Spot on, Sceadwin.
While awaiting your answers, I connected it up to the mains last night and 4 hours later both batteries were hot, and one was also leakiing. Fortunately the leakage was confined to the battery compartment, springs etc, easily cleanable. There may be some vapour spread from the hot liquid leaking out, but I can do nothing about that.
Not good engineering you're right, I've just added a Schottky 0.24V to stop the external source current flowing into the batteries, elementary, I should have thought of that.
Two further questions arise :
A. Are non-rechargeable batteries manufactured with precautionay venting or would that just be mechanical failure under internal pressure?
B. The unusual 3.05V zener voltage. I'll develop this point in detail as it may be of interest concerning aging of electronic equipment. This componant came from a box containing zeners recuperated in the 70's or 80's from old boards (TV,radio, cassette tape recorders etc). Selection by measuring with a 4.1V batt supply supply + 1k resistance, so we can say 3.05V at 1ma. There was a second zener, pratically identical in appearance, which came out at 3.4V, so less suitable. Here are its marking :
http://hpics.li/0ba2b7b
TFK BZY 6v2. TFK telefunken means it's old. It should be 6.2V. Something tells me that it must have been around 6.2V in the past or the circuit would have been rejected, not like a capacitor at half value which may go undetected. I googled on zener aging but got pratically nothing other than that their voltage does go down with the years, but to half value? Do you have any further knowlege on the subject?